1. Field of the Invention
The present invention relates to an electrophoretic display method and device in which charged migratory particles are migrated for display of an image.
2. Description of the Related Art
Recently, with rapid development of information equipment, the amount of data included in various kinds of information has increased more and more, and output of the information has been made in various forms. Generally, information is outputted in two primary ways, i.e., display-screen representation using a CRT or a liquid crystal, and hard-copy representation on paper using a printer or the like. In the display-screen representation, increasing needs exist for a display device that has low power consumption and is thin. Above all, a liquid crystal display has been actively developed and commercialized as a display device adaptable for such needs.
However, a current liquid crystal display has problems, which are not yet overcome to a satisfactory level, in that characters displayed on a screen become hard to perceive depending on the angle of viewing the screen and the presence of reflected light, and a burden is imposed on a viewer's visual organ due to, e.g., flickering and low luminance of a light source. Also, the display-screen representation using a CRT can provide the contrast and luminance at a satisfactory high level as compared with the case of using a liquid crystal display, but it accompanies flickering, etc. and hence also cannot be regarded as having a sufficient display quality as compared with the hard-copy representation described below. Additionally, the display-screen representation using a CRT entails a large and heavy body, and is therefore very poor in portability.
Meanwhile, at the beginning of the electronization era, it was thought that the hard-copy representation would no longer be required with the progress of electronization of information. In practice, however, a great deal of information is still outputted in the form of hard copies. The reasons are as follows. When information is displayed using a display unit, there occurs not only the above-mentioned problems with regard to display quality, but also another problem that a resolution achieved by the display-screen representation is generally about 120 dpi at maximum, which is fairly lower than that in the case of printing out information on paper (usually not lower than 300 dpi). Accordingly, the display-screen representation imposes a greater burden on a viewer's visual organ than the hard-copy representation. As a result, although information can be confirmed on a display screen, the information is often outputted in the form of hard copies. Another major reason why the hard-copy representation is utilized in spite of a capability of displaying information on a display screen, is that, unlike the display-screen representation, hard copies of information can be arranged side by side in large number without being restricted by a display size defining a display area, and they can be rearranged or checked in order with no need of complicated device operations. Furthermore, the hard-copy representation requires no energy for holding information in a represented state, and has superior portability enabling information to be read or checked in any place and at any time unless the amount of information is extremely large.
Thus, the hard-copy representation has various merits over the display-screen representation so long as moving images or frequent rewriting is not needed, but it is disadvantageous in consuming a great deal of paper. In recent years, therefore, a rewritable recording medium (i.e., a recording medium that enables an image to be displayed in many recording and erasing cycles with high viewability, but does not require energy for holding the image in a displayed state) has been actively developed. Such a third rewritable display system taking over superior characteristics of hard copies is herein called a paper-like display.
Requirements of the paper-like display are, for example, that it is rewritable, requires no or a sufficiently small amount of energy for holding an image in a displayed state (memory character), has superior portability, and has a high display quality. At present, one example of a display system, which can be regarded as the paper-like display, is a reversible display medium employing an organic low-molecular and high-molecular resin matrix and being able to record or erase an image by a thermal printer head (e.g., see Japanese Patent Laid-Open Nos. 55-154198 and 57-82086). Such a matrix is employed in display portions of some prepaid cards, but still has problems that the contrast is not so high and the number of times at which an image can be recorded and erased repeatedly is relatively small, i.e., on the order of 150 to 500.
As another display system capable of being utilized as the paper-like display, there is known an electrophoretic display device (U.S. Pat. No. 3,612,758) invented by Harold D. Lees, et al. Also, Japanese Patent Laid-Open No. 9-185087 discloses an electrophoretic display device. Such a display device comprises a disperse system wherein charged migratory particles are dispersed in a dielectric liquid, and a pair of electrodes is arranged in an opposing relation with the disperse system situated between the electrodes. By applying a voltage to the disperse system through the electrodes, charged migratory particles are attracted under electrostatic forces to the side of the electrode having a polarity opposite to that of charges of the migratory particles themselves based on the electrophoresis of charged particles. Display of information is performed by coloring the migratory particles and utilizing a difference between the color of the migratory particles and the color of the dyed dielectric liquid. More specifically, when the migratory particles are attracted onto the surface of a first electrode that is closer to the viewer and is light transparent, the color of the migratory particles is observed. On the contrary, when the migratory particles are attracted onto the surface of a second electrode that is farther away from the viewer, the color of the dielectric liquid, which is dyed so as to have different optical characteristics from those of the migratory particles, is observed.
In the above-described electrophoretic display device, however, a dye and a coloring material in the form of ions, for example, must be mixed in the dielectric liquid, and the presence of such a coloring material tends to act as an unstable factor in the electrophoretic operation because of giving rise to a new transfer of charges. This tendency may deteriorate the performance, useful life and stability of the display device.
To overcome the above problem, Japanese Patent Laid-open Nos. 49-5598 and 11-202804 propose a display device wherein a pair of electrodes, i.e., first and second driving electrodes, are arranged on the same substrate and migratory particles are migrated horizontally as viewed from the viewer. By applying voltages to the first and second driving electrodes, the migratory particles in a transparent dielectric liquid are horizontally migrated parallel to the substrate surface between the first and second driving electrodes based on the electrophoresis of charged particles, whereby an image is displayed.
In such an electrophoretic display device of the horizontally migrating type, the dielectric liquid is transparent and the first and second driving electrodes have different colors as viewed from the viewer side such that the color of one electrode coincides with the color of the migratory particles. Assuming, for example, that the color of the first driving electrode is black, the color of the second driving electrode is white, and the color of the migratory particles is black. The second driving electrode is exposed to provide a white view when the migratory particles are distributed over the first driving electrode, and the black color of the migratory particles is viewed when the migratory particles are distributed over the second driving electrode.
A display device comprising a large number of pixels arranged in a matrix pattern is electrically addressed in two primary ways, i.e., an active matrix mode and a passive matrix mode.
In the active matrix mode, a switching element such as a thin film transistor (TFT) is formed corresponding to each pixel, and voltages applied to the pixels are controlled in an independent manner for each pixel. By using the active matrix mode, the electrophoretic display device of the horizontally migrating type can be operated with a high display contrast. However, the active matrix mode has problems that the process cost is relatively high and it is difficult to form thin film transistors on a polymer substrate because of a high process temperature required in formation of the thin film transistors. These problems are particularly critical to manufacture of a paper-like display that is intended to be low in cost and flexible. A process for forming thin film transistors with a polymer material, which is adaptable for printing, is proposed to overcome those problems, but it is still an unknown quantity in practical applicability.
In the passive matrix mode, since only X-Y electrode lines are required as components necessary for addressing, the cost is relatively low and the electrodes lines can be easily formed on a polymer substrate. When applying a write voltage to a selected pixel, a voltage corresponding to the write voltage is applied to the X- and Y-electrode lines that cross each other at a point defining the selected pixel. In general, however, where an electrophoretic display device is operated by the passive matrix mode, there occurs so-called crosstalk, i.e., a phenomenon that the write voltage is applied to not only the selected pixel but also other pixels around it, whereby the display contrast is noticeably deteriorated. This is a problem that takes place inevitably because the electrophoretic display device does not have a definite threshold characteristic with respect to the write voltage.
To cope with the above-mentioned problem, it has been proposed to realize the passive matrix addressing in an electrophoretic display device, which does not have a threshold in principle, by employing a three-electrode structure wherein a control electrode is provided in addition to a pair of display electrodes. Most proposals regarding the three-electrode structure are related to electrophoretic display devices of the type using vertically arranged electrodes, as disclosed in, by way of example, Japanese Patent Laid-Open No. 54-085699 (corresponding to U.S. Pat. No. 4,203,106).
For a three-electrode structure in the electrophoretic display device of the horizontally migrating type, only one proposal is disclosed in Japanese Patent Publication No. (by PCT application) 8-507154 (corresponding to U.S. Pat. No. 5,345,251). However, a disperse solution used in Japanese Patent Publication No. (by PCT application) 8-507154 seems to be not transparent, but colored. This related art therefore differs in category from the electrophoretic display devices of the horizontally migrating type, which are featured by using a transparent disperse solution, as disclosed in the above-cited Japanese Patent Laid-Open Nos. 49-5598 and 11-202804 and as intended by the present invention.
Japanese Patent Publication No. (by PCT application) 8-507154 discloses two types of constructions (FIGS. 17A and 17B of the attached drawings). In the first construction (FIG. 17A), a control electrode 14 is arranged as a third electrode on the side of a second substrate 2 in an electrophoretic display device of the horizontally migrating type. In the second construction (FIG. 17B), a control electrode 14 is arranged as a third electrode between a first driving electrode 3 and a second driving electrode 4 both arranged on the side of a first substrate 1.
In any type of the first and second constructions, the first driving electrode 3 in the forked form as an assembly of a plurality of line electrodes and the second driving electrode 4 in the forked form as an assembly of a plurality of line electrodes, which are laid between adjacent lines of the first driving electrode 3, are both arranged on the first substrate 1 within an area of each pixel. The second driving electrode 4 is arranged on a step 15 formed by a thick chrome film. Accordingly, a level difference 22 of about 0.3 μm is formed at the boundary between the first driving electrode 3 and the second driving electrode 4. In the first construction, the control electrode 14 is formed on the underside of the second substrate 2 over the entire surface of each pixel area, the second substrate 2 being arranged in an opposing relation to the first substrate 1 with a spacing of 25 μm to 116 μm left between both the electrodes. In the second construction, the control electrode 14 is arranged on the first substrate 1 between respective lines of the first driving electrode 3 and the second driving electrode 4. In FIGS. 17A and 17B, for the sake of explanation, the first driving electrode 3 and the second driving electrode 4 are each illustrated as being constituted by one line.
The write operation of the electrophoretic display device disclosed in Japanese Patent Publication No. (by PCT application) 8-507154 will be described with reference to FIGS. 18 and 19. FIG. 18 shows migratory particles in respective operational conditions, and FIG. 19 shows applied pulses and a change of reflectance. The cell construction is the same as that shown in FIG. 17A (except for only one pixel being shown in FIG. 18).
Note that values of applied voltages mentioned in the following description are ones obtained under conditions of an experiment actually conducted by the inventors, and the conditions of the experiment are not exactly coincident with those described in Japanese Patent Publication No. (by PCT application) 8-507154. Such a discrepancy in those conditions primarily depends on differences in physical properties such as the polarity and amount of charges on migratory particles used. Hereunder, the values of applied voltages, which were obtained as results of the experiment made on the migratory particles used by the inventors, are employed for easier comparison with the operation of the present invention described later.
Also, although it seems that a colored liquid is used as a dielectric solution in Japanese Patent Publication No. (by PCT application) 8-507154, a transparent dielectric liquid is used in the following description for easier comparison with the operation of the present invention described later. Furthermore, for a method of developing display contrast, the following description is made as using a similar method to that employed in embodiments of the present invention wherein the color of the migratory particles is black, the color of the first driving electrode is black, and the color of the second driving electrode is white.
It is supposed that the migratory particles 6 are positively charged, the first driving electrode 3 serves as a common electrode, and a driving voltage Vd and a control voltage Vc are applied respectively to the first driving electrode 3 and the control electrode 14 with the ground potential of the second driving electrode 4 being as a reference.
In FIG. 8, a time period Ta represents a state where a white view is maintained. Also, arrows schematically indicate vectors of an electric field produced in a cell. The migratory particles 6 collected over the first driving electrode 3 are restrained from moving toward the side of the second driving electrode 4 due to the presence of the level difference 22 between the first driving electrode 3 and the second driving electrode 4. At the same time, the migratory particles 6 are held down to be urged toward the first substrate side under the control voltage Vc=+250 V applied between the first driving electrode 3 and the control electrode 14. During this time period Ta, therefore, the migratory particles 6 are stabilized in a condition as shown and a white view state with a reflectance R of about 70% is maintained. The driving voltage Vd=+5 V applied to the first driving electrode 3 in a state, in which a current view is maintained, serves to suppress a tendency of the migratory particles 6 near the level difference 22 to migrate toward the side of the first driving electrode 3 in the black view maintained state.
In a write period Tb, the driving voltage Vd=+50 V and the control voltage Vc=+50 V are applied. Since the first driving electrode 3 and the control electrode 14 are set to the same potential, the migratory particles 6 are released from being held down under the control voltage, whereby all of the migratory particles 6 are horizontally migrated toward the side of the second driving electrode 4 along the driving electrode surfaces beyond the level difference 22. As a result, the reflectance R abruptly decreases.
In a time period Tc representing a state in which a black view is maintained, the migratory particles 6 are held down to be urged toward the first substrate side as shown under the control voltage Vc=+250 V. Accordingly, a black view state with a reflectance R of about 5% is maintained.
The passive matrix addressing method disclosed in Japanese Patent Publication No. (by PCT application) 8-507154 will be described below with reference to FIGS. 20 and 21. Let us assume an electrophoretic display device of the horizontally migrating type has an (m×n) matrix wherein m columns of pixels are arrayed in the X-direction and n rows of pixels are arrayed in the Y-direction. Corresponding to the array configuration of pixels, a number m of data-signal electrode lines connected to the control electrodes 14 are arranged in the column direction, and a number n of scan-signal electrode lines connected to the first driving electrodes 3 are arranged in the row direction, with both the lines crossing each other in an orthogonal relation. The second driving electrode 4 is fixedly maintained at the ground potential so as to serve as a common electrode.
First, Vd=−50 V is applied to all of the scan-signal electrode lines and Vc=0 V is applied to all of the data-signal electrode lines so that all of the migratory particles 6 are collected over the first driving electrode 3 (FIG. 20A, total erasure). Then, the scan-signal electrode lines are selected one by one in sequence from the top in the Y-direction for writing. In a selection period (write period), Vd=+50 V is applied to the scan-signal electrode lines, Vc=+50 V is applied to those ones of the data-signal electrode lines corresponding to selected pixels, and Vc=+250 V is applied to the other ones of the data-signal electrode lines corresponding to non-selected pixels. For the selected pixels, the migratory particles 6 are migrated to the side of the second driving electrode 4 beyond the level difference under the driving voltage Vd=+50 V applied between the first and second driving electrode 3, 4, whereby writing is performed (FIG. 20B). For the non-selected pixels, the driving voltage Vd=+50 V is also applied to the first driving electrode 3. In the first construction, however, the migratory particles 6 are held down to be urged onto the first driving electrode 3 under the control voltage Vc=+250 V and are prevented from migrating (to perform writing) (FIG. 20C).
On the other hand, in a non-selection period, Vd=+5 V is applied to the scan-signal electrode lines, and Vc=+50 V or +250 V is applied to the data-signal electrode lines (FIGS. 21A to 21D). In any case, the migratory particles 6 are held down to be urged onto the surface of the first substrate as shown under the control voltage.
Thus, writing of information is performed by the passive matrix addressing method in the electrophoretic display device of the horizontally migrating type that does not have a threshold characteristic.
However, the following problems are experienced with the electrophoretic display device of the horizontally migrating type disclosed in Japanese Patent Publication No. (by PCT application) 8-507154.
The disclosed first construction has a limitation that the level difference 22 defined by the step 15 cannot be set to a large value. If the level difference is too large, part of the charged migratory particles 6 could not move over the level difference and would remain on the lower one of two surfaces defining the level difference when forced to migrate in the selection period, thus resulting in a reduced display contrast (FIG. 22A). To avoid the migratory particles 6 from remaining on the lower surface, the height of the step 15 must be limited to a value approximately equal to the diameter of the migratory particles 6.
Due to such a limitation imposed on the height of the step 15, the level difference cannot provide the effect of suppressing the migration of the migratory particles 6 at a sufficient level. Accordingly, when applying the control voltage Vc to hold down the migration of the migratory particles 6 for the non-selected pixel (FIG. 20C) in a condition where the driving voltage Vd is applied in the selection period, part of the migratory particles 6 moves over the level difference because of the step 15 being low. This phenomenon gives rise to a serious problem that crosstalk occurs and the display contrast deteriorates (FIG. 22B).
If the control voltage Vc is set to a sufficiently high value, the undesired migration of the migratory particles 6 can be prevented to a nearly satisfactory extent. However, this solution not only has the disadvantage of increasing the applied voltage, but also brings about another problem that charges injected into dielectric components of the device under a high voltage remain there even after release of the high voltage, and the operational condition of the migratory particles 6 becomes unstable due to an unintended electric field caused by the remaining charges.
The limitation imposed on the height of the step 15 raises still another problem as follows. Since the height of the step 15 is not sufficient, the area difference between the first driving electrode 3 and the second driving electrode 4 cannot be set to a large value. If the area difference is set to a large value, the migratory particles 6 would flow over onto the electrode surface having a larger area even when the migratory particles 6 are urged such that they are all collected on the electrode surface having a smaller area (FIG. 22C). Consequently, the display contrast is restricted because it is determined by an area ratio between the first driving electrode 3 and the second driving electrode 4.
Further, in the disclosed first construction (FIG. 17A), the effect of suppressing the migration of the migratory particles 6, provided by the level difference, is restricted only in the direction toward the higher surface side from the lower surface side, whereas the migration of the migratory particles 6 from the higher surface side to the lower surface side is rather accelerated. The write direction is therefore limited to only one direction from a white to black view. In other words, the addressing method for writing is restricted to the steps of first collecting the migratory particles 6 for an overall screen to the lower surface side for total reset, and then writing information by migrating the migratory particles 6 in one direction to the higher surface side. It is hence impossible to perform bi-directional writing, i.e., black-to-white and white-to-black writing, and to realize such an operation as selectively rewriting only part of an image on the screen.
The disclosed second construction (FIG. 17B) operates in the selection period such that a high voltage is applied to the control electrode 14 for the non-selected pixel to prevent the migratory particles 6 from moving in both directions, and the voltage of the control electrode 14 is set to 0 V for the selected pixel, allowing the migratory particles 6 to smoothly migrate in either direction. In this case, therefore, the step 15 is considered to not be an essential component.
In the disclosed second construction, however, the control electrode 14 is able to control the migration of the migratory particles 6 only between the first and second driving electrodes, and is unable to control the migration of the migratory particles 6 within each of the driving electrode surfaces. Due to a control voltage applied to the control electrode 14 in the non-selection period, therefore, the migratory particles 6 having been evenly dispersed over the driving electrode surface are repellently migrated in a direction away from the control electrode 14 and are partially distributed within the driving electrode surface as shown in FIG. 23A or 23B. This invites a problem of noticeably reducing the display contrast.